Production of electronic modules typically involves an electrical circuit patterned on a fiberglass/epoxy, ceramic, or flexible polymer substrate with copper or cermet conductors. The electrical functions are imparted through circuit components (i.e., transistors, resistors, capacitors, diodes, microprocessors) which are soldered to the surface of the board (Surface Mount Technology) or soldered in holes through the board (Through Hole Technology). Constructions of this sort are widely used in many industries: automotive, telecommunications, entertainment, military and computers. Automotive applications of circuit boards with soldered components include engine control modules, ignition modules, instrument panel controls, radios, electrical centers, and various other accessory and convenience modules.
The leading technique used throughout the electronics industry for soldering components to the substrate uses a metallic solder alloy containing, by weight, 63% tin/37% lead. It is applied to the circuit board either as a paste which is heated to more than 200.degree. C. to "reflow" the paste into a solder joint. Alternatively, the board is passed over a molten wave of solder to form joints to bond the electrical components to the circuit board. In either case, a flux material consisting of weak acids is used to remove surface oxidation from metallic surfaces and allow the molten solder to bond to the surfaces and form reliable solder joints.
While this solder attachment technology has existed for many decades, it does have some notable shortcomings. One issue is the lead in the alloy. Lead has already been banned from paint, gasoline, and plumbing solders for environmental and safety reasons. Numerous environmental regulations have been proposed to tax, limit, or ban the use of lead in electronic solders. A second shortcoming is the use of the above mentioned flux material for removing surface oxides. This flux leaves a residue on the finished parts that must be cleaned off with a solvent spray. This is an expensive and often inefficient process. In addition to lead and flux, the solder needs to be processed at temperatures above 200.degree. C. This temperature often dictates the use of an expensive substrate in order to withstand the soldering process temperature, even though the assembly will never encounter temperatures nearly as high in the rest of its service life. In certain high temperature automotive applications (i.e., engine compartment locations where temperature can reach 175.degree. C.), the conventional tin/lead solder may not be usable since the service temperature is dangerously close to the melting point of the alloy. Yet another shortcoming of solder is that the metallic alloy is a brittle material that can crack after repeated thermal cycling. In cases where expansion rates of component and substrate are vastly different, cracked solder joints may be a significant problem.
There are two main alternatives to the existing tin/lead solders. One is a lead-free metallic solder alloy and the other is an electrically conductive synthetic resin adhesive. In the family of metallic solders, many lead free alloys exist including, tin, silver, indium, bismuth, copper and antimony among other metals. Numerous research efforts have evaluated lead free alloys, but have found no lead free solders that directly match the properties of the existing 63% tin/37% lead alloy in use today. Issues for lead free solders include: higher process temperature (which may require redesigned circuit boards and electrical components), different mechanical properties, longer processing times and more sensitivity to assembly process parameters.
The second alternative, electrically conductive adhesives, offer several advantages over traditional solder assembly including: absence of lead, low processing temperatures, no need for solder flux or subsequent flux cleaning steps, improved mechanical properties, better high temperature performance, and a simplified assembly process. Conductive adhesives have been on the market for several decades and are widely used in sealed semiconductor packages. However, use of conductive adhesives for unsealed circuit boards represents a new application for adhesives.
Several international research efforts (IVF-Sweden, Delta-Denmark) have evaluated conductive adhesives as a solder replacement. They have reported successful results for niche applications but have not identified a drop-in solder replacement. The technology is limited by electrical resistance stability through temperature/humidity aging and impact strength.
In the US, the National Center for Manufacturing Sciences (NCMS) performed an extensive evaluation of electrically conductive adhesives for surface mount printed circuit applications. In that cooperative industry project, over 30 commercially available adhesives were evaluated for basic electrical and mechanical properties. The NCMS team defined a test method for evaluating electrical resistance of a conductive adhesive joint as well as an impact test to assess the capability of these adhesives for holding a component on a circuit board during an impact (S. L. McCarthy, "New Test Methods for Evaluating Electrically Conductive Adhesives," J. Surface Mount Technology, Vol. 9, July 1996, pp. 19-26). The electrical testing was performed before and after exposure to an elevated temperature/humidity environment (85.degree. C., 85%RH) and was conducted with copper parts and tin/lead parts. The testing revealed that some adhesives had adequate electrical resistance when copper surfaces were used. On the other hand, no adhesives were identified for producing adequate resistance with tin/lead surfaces. Impact testing also concluded that no adhesives were capable of meeting the NCMS impact test requirement. The use of present conductive adhesives for surface mount component attachment to printed circuit boards is very limited because the impact strength and electrical resistance stability that they provide has fallen far short of the industry standard tin/lead solder performance.
Previous testing of commercially available adhesives has concluded that conductive adhesives are suitable for only niche applications, limited by resistance and impact requirements. Contact with commercial adhesive vendors has revealed that most have been stopped by the requirement for resistance stability on Sn/Pb surfaces. Some vendors have claimed success at developing an impact resistant adhesive, but none have been able to address the resistance variability when in contact with tin/lead layers. In fact, many adhesive vendors have acknowledged that impact strength and resistance stability are mutually exclusive parameters. The conductive adhesives of this invention provide formulations with both impact and resistance properties that are comparable to or exceed the performance of tin/lead solders. This combination of properties expands the window of application for conductive adhesives from limited niche applications to potential universal solder replacement in circuit board applications.
From a traditional viewpoint, cured epoxy resins are often thought of as rigid and brittle materials. This rigidity and brittleness are further magnified when fillers are added to accomplish certain desirable properties such as in the case of metal filled epoxy resins. Conventional epoxies filled with 70% to 80% silver flakes are highly conductive but very brittle and failure occurs even under a mild mechanical shock condition.